Cr50: Export the chip-specific SPI master functions
Cr50: Export the chip-specific SPI master functions

This change is motivated by an internal use case.

BRANCH=none
BUG=none
TEST=make buildall -j
Successfully used the exported functions on Cr50.

Change-Id: I5a54b4ea407866c7d7a4bd075d7773ac81e00930
Signed-off-by: Nadim Taha <b42c84a30622c1b59fd137ddd0faa205f9b90636@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/348215
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec